Experiments are proposed to quantitate immunoglobulin genes by RNA/DNA hybridization. Microsomes from hyperimmune spleens and myeloma tumors by mice will be used as a source of RNA, rich in immunoglobulin messenger RNA and largely devoid of other mRNA species. The proteins coded for by mRNA in membrane bound ribosomes are secreted into the cisternae; therefore, an estimate of the microsomal mRNAs contaminating immunoglobulin mRNA can be made by determining which proteins are contained in the microsomal cisternae besides immunoglobulin. RNA will also be extracted from nonlymphatic organs which produce the same proteins which "contaminate" spleen microsomes. Such RNA will be used in RNA/DNA hybridization reactions to compete for all RNA species in spleen microsomes other than immunoglobulin mRNA. Similarly microsomal RNA from immunoglobulin-producing myelomas will be hybridized to DNA in competition with RNA of a nonproducer subline of the same tumor. Our preliminary experiments have indicated that the haploid genome contains enough DNA hybridizing with immunoglobulin mRNA to code for six to ten thousand proteins of the size of the variable region of immunoglobulin polypeptide chains. We propose various experiments to check the validity of these findings and in addition to determine whether genes for the variable and constant regions of immunoglobulins are somatically amplified.